Evidence for a Dirac Spectrum in the Topological Insulator Bi$_2$Te$_2$Se from High-Field Shubnikov-de Haas Oscillations

The transport properties of surface states in the 3D topological insulators based on bismuth have been observed in a number of experiments. However, there is still no direct evidence for the Dirac dispersion predicted for these states. We have measured the Shubnikov-de Haas (SdH) oscillations in Bi$_2$Te$_2$Se in intense dc fields. At B$>$40 T, we can reach the n=1 surface Landau Level. In the index plot (of 1/B versus n), the relatively large oscillation amplitudes in our crystals (as large as $17\%$ of the total conductance) enables us to resolve, in the high-field limit, the $\frac{1}{2}$-shift predicted from the Berry phase in the Dirac spectrum. In addition, the linearity of the index plot shows that the surface Lande g-factor is quite small (less than 5) in Bi$_2$Te$_2$Se, in contrast with recent inferences based on low-B SdH experiments. \\ Supported by NSF DMR 0819860